The present invention provides novel compositions for Synthetic Lightweight Aggregates (SLA) and methods for their use in construction applications such as geotechnical lightweight fill, concrete masonry blocks, lightweight concrete structures and asphalt paving. The SLA is produced by melt compounding high concentrations of fly ash from coal into a combination of various thermoplastics. The SLA""s are preferably comprised of low or negative value raw materials such as high carbon fly ash and post consumer waste.
Landfill space is at a premium and alternatives such as recovery, recycling and reuse of wastes as beneficial products should be strongly encouraged and examined. One potentially strong and viable market is to develop recyclable wastes into granular aggregates, a common material used throughout the construction industry.
The reuse of waste materials in building and highway construction has been a growing phenomenon since the early 1980""s. The high demand for construction materials and building products makes them a favorable medium in which to reuse recyclable materials.
Aggregates are used in a variety of building and construction applications. Annually, approximately 2000 million tons of granular material are used in the US. This is the largest quantity of material used in any industry. Almost all aggregates are produced from natural resources such as gravel pits (glaciated), river beds (alluvial), and rock quarries. In addition to depleting these natural resources, mining for aggregates poses serious environmental implications such as the disturbance of natural habitats.
In the context of concrete formulations, aggregates are fine or coarse particles consisting of sands, crushed stones and gravels that are mixed with the cement paste to form a concrete mixture. Most conventional concrete structures are produced using xe2x80x9cnormal weightxe2x80x9d sand and gravel aggregate. However, there are a number of applications where the relatively high specific gravity of the normal weight aggregate is an undesirable characteristic. In applications where a lighter weight concrete is required, the normal weight aggregate may be partially or totally replaced with lower specific gravity or xe2x80x9clightweightxe2x80x9d aggregate. Lightweight concrete is commonly used in applications such as concrete masonry units (i.e. concrete blocks) or bridge decks. Lightweight aggregates can be naturally occurring but they are relatively scarce. Most of the lightweight aggregate material that is used for lightweight concrete is xe2x80x9cmanufacturedxe2x80x9d by some means. For example, thermal treatment or pyroprocessing of certain naturally occurring minerals can produce an aggregate having a cellular or foam-like structure, hence a lower bulk specific gravity. Expanded clay is an example of a lightweight aggregate. Compared to normal weight aggregate, that usually has a bulk specific gravity greater than 2.6, a lightweight aggregate has a specific gravity less than 2.2. Concrete that is produced using lightweight aggregate will have a lower specific gravity than that produced with normal weight aggregate, but can still have mechanical properties suitable for structural applications.
The concept of using plastic powders or granules as a concrete additive or aggregate is not new. Shuhnan (U.S. Pat. No. 5,580,378) used micronized polystyrene foam particulates (or other plastic particulates) as a lightweight concrete additive. Schroeder in Public Roads, 58 (1994), reports on a study that used HDPE as a partial replacement for fine aggregate in a lightweight concrete mix. The concrete containing the HDPE granules had a lower compressive strength but greater toughness. Large lightweight concrete blocks produced with reground (waste) plastic aggregate are even commercially available.
Likewise, the concepts of using fly ash as an additive (usually as a filler) or as a concrete mineral additive are also not new. Fly ash is a fine ash byproduct commonly produced by the combustion of coal during the generation of electrical power. The fly ash is separated from the hot flue gases before it escapes into the atmosphere. Fly ash is commonly classified as either class C or F (depending on its source-anthracite, bituminous or lignite, subbituminous coal, respectively), and based on its unburned carbon content (ASTM C618 xe2x80x9cFly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Portland Cement Concretexe2x80x9d). Low carbon fly ash (LCFA) is commonly used as a partial replacement for Portland cement in concrete, or in flowable fill applications. High carbon fly ash, having carbon content greater than about 6%, is much less marketable as a secondary material since free carbon is an undesirable contamination in concrete. Fly ash is also used as a mineral filler in plastics, however, in much smaller volumes than the construction applications described above. The results of many experimental studies conducted with fly ash have shown that the addition of fly ash filler does increase the stiffness of a plastic formulation, but like most fillers, reduces impact resistance. Fly ash offers a significant economic advantage over competing fillers such as calcium carbonate, but does tend to impart a grayish color to the plastic formulation (to a degree dependent on the unburned carbon concentration).
Approximately 29 million tons of plastics (or polymers) are generated annually for use in the US. Close to half of this amount enters the municipal solid waste (MSW) stream with plastics representing upwards to 20% of a MSW landfill""s volume. The majority of waste plastics originate from packaging and containers. Most curbside recycling programs in the United States recycle only the polyethlene terephalate (PET) and the high density polyethylene (HDPE) portions of their collected plastics. The plastics that are not recycled (and the portion of the coal combustion byproducts that are not utilized) are landfilled.
Plastic recycling efforts have increased substantially over the last decade but are still insufficient. For example, high density polyethylene (HDPE) is a very common plastic used in making containers such as milk jugs, shampoo bottles and soft drink bottles. However, only 25% of HDPE is recycled, and, worse still, only 1 to 2 percent of all plastics produced are recycled. One factor which influences this recycling rate is the need to separate plastics by type. There are a variety of plastic types (e.g., polypropylene, polystyrene and polyethylene, etc.), however, a mixture of recycled types of plastic will result in a product which exhibits physical and structural behavior inferior to its parent materials. Other factors which contribute to this low recycling rate including the relatively low price of oil (from which plastics are derived), the available recycling market, and the comparative cost for land fill disposal of plastics.
The preferred plastic material for use in the synthetic lightweight aggregate should be available in large quantities such as post consumer wastes which has little or no resale value. The most likely material candidate for such an application is mixed waste plastics from both municipal and industrial waste streams. One possible material source is the relatively low-value post consumer PCCS # 3-7 stream. This material currently has very limited application since any attempt to use different plastic resins together in a melt blending process requires extensive compatibilizers especially if more than two resins are involved. The numbers referred to here are developed by the Society for the Plastic Industry (SPI) in order to assist consumers and sorters at post consumer waste recycling facilities. A number ranging from 1 to 7 inside a recycling symbol has been assigned by the SPI to each of the major plastic resin types and that number is molded onto each plastic product.
A bale of recycle plastics described as PCCS # 3-7 generally consists of many different thermoplastic materials collected as part of a recycling program after many of the HDPE and PET bottles and containers had been removed for recycling. This material is routinely sent to landfills or incinerators by the material recovery facilities as waste.
What is needed is a lightweight aggregate comprising materials of low or negative commercial value which can be combined into materials of construction for building or surfaces and provide equivalent, or in some cases, improved physical properties at reduced weight or specific gravity.
The present invention is directed at lightweight synthetic aggregates (SLA""s) comprised of various combinations of low or negative value materials, namely fly ash and multipolymer component mixtures (i.e. two or more polymer components), particularly post consumer plastic waste. When used in construction materials, such as concrete and concrete blocks, geotechnical applications (such as soil) and asphalt surfacing, the result is a reduction in weight with little reduction in properties when compared to the use of a conventional natural resource aggregate. Additionally, a valuable use for low value materials has been created.
With respect to the construction industry and the engineering profession, these new and innovative construction materials may not only be more economically advantageous than traditional (natural) granular materials but may also out-perform them. In addition, the production of this new material allows the recovery and reuse of two large-quantity, raw materials from the waste stream (fly ash and waste plastics), thus, satisfying the goals of sustainable development in which wastes are minimized, their reuse maximized, and natural resources preserved.
It is therefore the object of this invention to provide synthetic lightweight aggregate comprising blends of fly ash and multiple polymer components mixtures in ratios up to 90/10 (fly ash/mixed plastic).
It is a further object to provide useful construction compositions using synthetic lightweight aggregate in soil, concrete and asphalt applications.
It is a still further object of this invention to provide a simple processing method to provide synthetic lightweight aggregate which does not require additional compatibilizing agents in order to handle multiple types of plastics at once.
It is still a further object of the invention to provide a concrete composition using synthetic lightweight aggregate having improved ductility and lower weight.
It is an additional object of this invention to provide a use for low value materials such as high carbon fly ash and post consumer and municipal plastic waste, without having to separate the various plastics by type.